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Abstract:

Disclosed herein is a porous silicon-based electrode active material,
comprising a silicon phase, a SiOx (0<x<2) phase and a silicon
dioxide phase and having a porosity of 7-71%.

Claims:

1. A porous silicon-based electrode active material comprising a silicon
phase, a SiOx (0<x<2) phase and a silicon dioxide phase and
having a porosity of 7-71%.

2. The porous silicon-based electrode active material of claim 1, wherein
the silicon dioxide phase is dispersed in the SiOx phase and
crystalline, and it comprises cristoballite and is present in an amount
of 2-50 wt % based on the weight of the silicon-based anode active
material.

3. The porous silicon-based electrode active material of claim 1, wherein
x in the SiOx is 0.5-1.2 or the SiOx is silicon monoxide.

5. The porous silicon-based electrode active material of claim 1, wherein
a concentration of the silicon phase is higher center portion than
center-periphery portion of the electrode active material and a
concentration of the silicon dioxide phase is higher center-periphery
portion than center portion of the electrode active material.

6. The porous silicon-based electrode active material of claim 1, wherein
the silicon phase and the silicon dioxide phase is formed by
disproportionation of SiOx (0<x<2).

7. A secondary battery, which comprises a cathode comprising a cathode
active material, a separator, an anode comprising an anode active
material, and an electrolyte, wherein the cathode active material or the
anode active material comprises the electrode active material of any one
of claims 1 to 6.

8. The secondary battery of claim 7, wherein the cathode active material
or the anode active material further comprises one or more selected from
the group consisting of graphite, soft carbon, hard carbon, and lithium
titanium oxide.

9. A method for preparing a porous silicon-based electrode active
material, the method comprising: mixing a fluorine-based solution and a
metal precursor solution and bringing the mixture into contact with
SiOx (0<x<2)-containing particles, thus electrodepositing
metal particles on the surface of the SiOx-containing particles;
bringing the metal particle-electrodeposited, SiOx-containing
particles into contact with an etching solution, thus etching the
SiOx-containing particles; bringing the etched SiOx-containing
particles into contact with a metal removing solution, thus removing the
metal particles; mixing the SiOx-containing particles, from which
the metal particles have been removed, with a solution of an alkaline
hydroxide in a polar solvent; and evaporating the polar solvent from the
SiOx-containing particles, and then heat-treating the
SiOx-containing particles.

10. The method of claim 9, wherein the fluorine-based solution is one or
more selected from the group consisting of hydrogen fluoride (HF),
fluorosilicate (H2SiF6) and ammonium fluoride (NH4F) and
the metal precursor solution comprises one or more selected from the
group consisting of silver, gold, platinum and copper and the
fluorine-based solution and the metal precursor solution are mixed with
each other at a volume ratio of 10-90: 90-10.

11. The method of claim 9, wherein the SiOx-containing particles are
used in an amount of 0.001-50 parts by weight based on 100 parts by
weight of the mixed solution of the fluorine-based solution and the metal
precursor solution.

12. The method of claim 9, wherein the etching occurs under the metal
particles of the metal particle-electrodeposited, SiOx-containing
particles.

13. The method of claim 9, wherein the etching solution is a mixed
solution of hydrogen fluoride (HF) and hydrogen peroxide (H2O2)
and the hydrogen fluoride (HF) and hydrogen peroxide (H2O2) are
mixed with each other at a volume ratio of 10-90: 90-10.

14. The method of claim 9, wherein the metal removing solution is one or
more selected from the group consisting of nitric acid, sulfuric acid and
hydrochloric acid.

15. The method of claim 9, wherein the alkaline hydroxide is one or more
selected from the group consisting of LiOH, NaOH, KOH, Be(OH)2,
Mg(OH), Ca(OH)2, and hydrates thereof.

16. The method of claim 9, wherein the SiOx, from which metal
particles have been removed, is mixed in an amount of 0.01-30 wt % based
on the weight of the alkaline hydroxide.

17. The method of claim 9, wherein the heat treatment is carried out at
750-1000.degree. C. for 5-120 minutes.

18. The method of claim 9, wherein the SiOx-containing particles are
disproportionated into silicon and silicon dioxide by the heat treatment.

19. The method of claim 9, wherein the method further comprises filtering
the heat-treated SiOx-containing particles.

20. The method of claim 9, wherein the method further comprises coating
the heat-treated SiOx-containing particles with carbon.

21. The method of claim 20, wherein the carbon is used in an amount of
1-30 wt % based on the total weight of the electrode active material.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the priority of Korean Patent Application
No. 10-2012-0080504 filed on Jul. 24, 2012 in the Korean Intellectual
Property Office, the disclosure of which is incorporated herein by
reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a porous silicon-based electrode
active material and a secondary battery comprising the same.

[0004] 2. Description of the Prior Art

[0005] Since the discovery of electricity in the 1800s, primary batteries
have developed into secondary batteries, and batteries having low
operating voltage have developed into batteries having high operating
voltage. Among this variety of batteries, lithium secondary batteries are
leading 21st battery technology and are receiving attention as energy
storage systems for a variety of applications, including mobile phones
and electric vehicles.

[0006] Lithium secondary batteries are energy storage devices in which
lithium ions move from the anode (negative electrode) to the cathode
(positive electrode) during discharge and move from the cathode to the
anode during charging when storing energy in the batteries. The lithium
secondary batteries have high energy density and low self-discharge rate
compared to other types of batteries, and thus are used in a wide range
of applications.

[0007] General lithium secondary batteries comprise a cathode, an anode,
an electrolyte and a separator. In early lithium secondary batteries,
lithium metal was used as the anode active material, but was replaced
with carbon-based materials such as graphite, because of safety concerns
resulting from the repeated charge/discharge cycles. The potential of the
electrochemical reaction of the carbon-based anode active material with
lithium ions is similar to that of lithium metal, and the change in the
crystal structure thereof during the intercalation/deintercalation of
lithium ions is low. Thus, the carbon-based anode active material can be
repeatedly charged and discharged and has excellent charge/discharge
cycle characteristics.

[0008] However, in recent years, as the lithium secondary battery market
has expanded from small-sized lithium secondary batteries for mobile
devices to large-sized lithium secondary batteries for automobiles, there
is a newfound need for a technology that can achieve the high capacity
and high output of anode active materials. Thus, non-carbon-based anode
active materials, including silicon, tin, germanium, zinc and lead-based
materials, have been actively developed, which theoretically have
capacities higher than carbon-based anode active materials.

[0009] Among these, silicon-based anode active materials have a capacity
of 4190 mAh/g, which is 11 times higher than the theoretical capacity
(372 mAh/g) of the carbon-based anode active materials, and thus have
received attention as a substitute for the carbon-based anode active
materials. However, in the case of using silicon alone as the anode
active material, its volume expands by a factor of 3 or more when it is
intercalated by lithium ions. For this reason, the battery capacity
decreases as the number of charge/discharge cycles increases, and safety
concerns also arise. Thus, in order to commercially use silicon as an
anode active material, many studies are required into that battery.

[0010] As a result, studies on silicon-based composites have been actively
conducted. Among these, studies have been made into the use of a
silicon-based material in combination with a carbon-based material. This
method was developed to minimize the volume expansion of the silicon
active material in order to increase capacity and charge/discharge cycle
characteristics. The most fundamental method for synthesizing the
composite is to coat the silicon-based material with carbon. This
improves the electrical conductivity between active material particles
and the electrochemical properties and the properties of the
electrochemical reaction with an electrolyte and reduces the volume
expansion of the silicon-based particles, resulting in an increase in the
battery lifetime. However, there is a problem in that the initial
charge/discharge efficiency is deteriorated due to the formation of an
irreversible phase by the silicon-based material during initial
charge/discharge cycling.

SUMMARY OF THE INVENTION

[0011] It is an object of the present invention to provide a porous
silicon-based electrode active material for a secondary battery, which
can improve the initial charge/discharge efficiency and capacity
maintenance rate of the secondary battery and reduce the thickness change
rate of the secondary battery to improve the lifetime characteristics.

[0012] The present invention provides a porous silicon-based electrode
active material, which comprises a silicon phase, a SiOx
(0<x<2) phase and a silicon dioxide phase and has a porosity of
7-71%.

[0013] The present invention also provides a secondary battery, which
comprises a cathode comprising a cathode active material, a separator, an
anode comprising an anode active material, and an electrolyte, wherein
the cathode active material or the anode active material comprises a
silicon-based electrode active material comprising a silicon phase, a
SiOx (0<x<2) phase and a silicon dioxide phase and having a
porosity of 7-71%.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 depicts a scanning electron microscope (SEM) photograph
showing the shape and crystal structure of a porous silicon-based
electrode active material (FIG. 1a) according to one embodiment of the
present invention and the results of X-ray diffraction (XRD) analysis of
the porous silicon-based electrode active material (FIG. 1b).

DETAILED DESCRIPTION OF THE INVENTION

[0015] The present invention provides a porous silicon-based electrode
active material comprising a silicon phase, a SiOx (0<x<2)
phase and a silicon dioxide phase and having a porosity of 7-71%.

[0016] In a porous silicon-based electrode active material according to
one embodiment of the present invention, the silicon dioxide phase may be
dispersed in the SiOx phase, and the silicon phase and the silicon
dioxide phase may be crystalline.

[0017] In a porous silicon-based electrode active material according to
one embodiment of the present invention, x in the SiOx may be
greater than 0 but smaller than 2, preferably 0.5-1.2, and the SiOx
may be commercially easily available silicon monoxide (SiO). If the value
of x is less than 0.5, the initial charge/discharge efficiency of the
battery will be high, but the amount of oxygen which can suppress the
volume expansion of the electrode will be small, and thus the lifetime of
the electrode will be reduced and the degree of suppression of the
expansion in the thickness of the electrode will be low, even though the
electrode active material has a porous structure, and if the value of x
is more than 1.2, the amount of oxygen will increase to reduce the
initial charge/discharge efficiency of the battery.

[0018] The silicon dioxide phase may include cristoballite and may be
present in an amount of 2-50 wt % based on the weight of the anode active
material. If the amount of the silicon dioxide phase is less than 2 wt %,
an increase in the initial charge/discharge efficiency of the battery
will not be enough, and if it is more than 50 wt %, the initial
discharge/discharge efficiency will increase, but the discharge capacity
of the battery will decrease.

[0019] Moreover, the porosity of a porous silicon-based electrode active
material according to one embodiment of the present invention may be
7-71% based on the total volume of the porous silicon-based electrode
active material, the porous silicon-based electrode active material may
have honeycomb-shaped pores formed on at least the surface or the surface
and inside of the porous silicon-based electrode active material and may
be composed of particles. If the porosity of the electrode active
material is less than 7%, the volume expansion of the electrode active
material during charging/discharging will not be suppressed, and if it is
more than 71%, the mechanical strength of the electrode active material
will be reduced due to the presence of a large amount of pores in the
electrode active material, and thus the electrode active material may
break down during battery fabrication processes (slurry mixing, pressing
after coating, etc.).

[0020] The particle size of the electrode active material may range from
several tens of nm to several tens of μm preferably 100 μm-50
μm.

[0021] In addition, in the electrode active material, the silicon and the
silicon dioxide may be present as nanometer-sized crystals in the active
material particles. Herein, the crystalline silicon has a size of 1-1000
μm, and the crystalline silicon dioxide has a size of 1-1000 μm.
The silicon phase, the SiOx phase and the silicon dioxide phase,
which are included in the electrode active material, can intercalate and
deintercalate lithium.

[0022] The silicon phase and the silicon dioxide phase may be formed by
disproportionation of SiOx (0<x<2). The concentration of the
silicon phase in the porous silicon-based electrode active material is
higher center portion than center-periphery portion of the electrode
active material, and the concentration of the silicon dioxide phase is
higher center-periphery portion than center portion of the electrode
active material. The "center portion" refers to a portion inside a line
corresponding to 50% of the maximum value of the length of the electrode
active material in the vertical direction from a line that is tangent to
the electrode active material, and the "center-periphery portion" refers
to a portion outside the line corresponding to 50%. In addition, "a
concentration higher center portion than center-periphery portion of the
electrode active material" means that the average concentration in the
portion inside the line corresponding to 50% of the maximum value of
diameter of the electrode active material is higher than the average
concentration in the portion outside the line.

[0023] When a porous silicon-based electrode active material according to
one embodiment of the present invention is used as an electrode active
material, lithium ions (Li+) which are intercalated into the anode during
the initial charge/discharge of the battery will react with silicon-based
oxide to form an irreversible phase such as lithium oxide or lithium
silicon oxide, in which the irreversible phase surrounds the silicon of
the silicon oxide or the lithium silicon oxide, thus inhibiting the
cracking or pulverization of the electrode active material. In addition,
because pores are present on at least the surface or the surface and the
inside of the electrode active material, they can improve the battery
capacity and can efficiently control the change in volume of the battery
during charge/discharge cycles, thus improving the lifetime of the
battery. Although an electrode active material according to one
embodiment of the present invention can be used as both a cathode active
material and an anode active material, it may preferably be an anode
active material.

[0024] The present invention also provides a secondary battery, which
comprises a cathode comprising a cathode active material, a separator, an
anode comprising an anode active material, and an electrolyte, wherein
the cathode active material or the anode active material comprises a
porous silicon-based electrode active material comprising a silicon
phase, a SiOx (0<x<2) phase and a silicon dioxide phase and
having a porosity of 7-71%.

[0025] The electrode active material according to one embodiment of the
present invention may be used in a secondary battery by mixing with a
typically used electrode active material, and the typically used
electrode active material may be one or more selected from the group
consisting of graphite, soft carbon, hard carbon, and lithium titanium
oxide.

[0026] The prepared electrode active material, specifically an anode
active material, may be prepared as an anode by using a preparation
method typically used in the art. For example, the anode active material
of the present invention is mixed and stirred with a binder, a solvent,
and a conductive material and a dispersant if necessary to prepare
slurry, and then an anode may be prepared by coating a collector with the
slurry and pressing.

[0028] N-methyl-2-pyrrolidone, acetone, or water may be used as the
solvent.

[0029] The conductive material is not particularly limited so long as it
does not cause chemical changes in the battery and has conductivity. For
example, graphite such as natural graphite or artificial graphite; carbon
black such as acetylene black, Ketjen black, channel black, furnace
black, lamp black, and thermal black; conductive fibers such as carbon
fibers or metal fibers; metal powders such as fluoro carbon powder,
aluminum powder, and nickel powder; conductive whiskers such as zinc
oxide whiskers and potassium titanate whiskers; conductive metal oxides
such as titanium oxide; and conductive materials such as polyphenylene
derivatives may be used as the conductive material.

[0030] An aqueous-based dispersant or an organic dispersant such as
N-methyl-2-pyrrolidone may be used as dispersant.

[0031] Similar to the preparation of the foregoing anode, a cathode active
material, a conductive material, a binder, and a solvent are mixed to
prepare a slurry, and then a cathode may be prepared by directly coating
a metal collector with the slurry or by casting the slurry on a separate
support and laminating a cathode active material film separated from the
support on a metal collector.

[0032] Examples of the cathode active material may be a layered compound,
such as lithium cobalt oxide (LiCoO2) or lithium nickel oxide
(LiNiO2), or a compound substituted with one or more transition
metals; lithium manganese oxides such as Li1+yMn2-yO4
(where y is 0 to 0.33), LiMnO3, LiMn2O3, and LiMnO2;
lithium copper oxide (Li2CuO2); vanadium oxides such as
LiV3O8, LiFe3O4, V2O5, and
Cu2V2O7; nickel (Ni)-site type lithium nickel oxide
expressed by a chemical formula of LiNi1-yMyO2 (where M is
cobalt (Co), manganese (Mn), aluminum (Al), copper (Cu), iron (Fe),
magnesium (Mg), boron (B), or gallium (Ga), and y is 0.01 to 0.3);
lithium manganese complex oxide expressed by a chemical formula of
LiMn2-yMyO2 (where M is Co, Ni, Fe, chromium (Cr), zinc (Zn),
or tantalum (Ta), and y is 0.01 to 0.1) or Li2Mn3MO8
(where M is Fe, Co, Ni, Cu, or Zn); LiMn2O4 having a part of
lithium (Li) being substituted with an alkaline earth metal ion; a
disulfide compound; and Fe2(MoO4)3. However, the cathode
active material is not limited thereto.

[0033] A typical porous polymer film used as a typical separator, for
example, a porous polymer film prepared from a polyolefin-based polymer,
such as an ethylene homopolymer, a propylene homopolymer, an
ethylene/butene copolymer, an ethylene/hexene copolymer, and an
ethylene/methacrylate copolymer, may be used alone or in a lamination
therewith as the separator. A typical porous nonwoven fabric, for
example, a nonwoven fabric formed of high melting point glass fibers or
polyethylene terephthalate fibers may be used, but the separator is not
limited thereto.

[0034] In an electrolyte solution used in the present invention, a lithium
salt, which may be included as an electrolyte, may be used without
limitation so long as it is typically used in an electrolyte solution for
a secondary battery. For example, one selected from the group consisting
of F.sup.-, Cl.sup.-, I.sup.-, NO3.sup.-, N(CN)2.sup.-,
BF4.sup.-, ClO4.sup.-, PF6.sup.-,
(CF3)2PF4.sup.-, (CF3)3PF3.sup.-.
(CF3)4PF2.sup.-, (CF3)5PF.sup.-,
(CF3)6P.sup.-, CF3SO3.sup.-,
CF3CF2SO3.sup.-, (CF3SO2)2N.sup.-,
(FSO2)2N.sup.-, CF3CF2(CF3)2CO.sup.-,
(CF3SO2)2CH.sup.-, (SF5)3C.sup.-,
(CF3SO2)3C.sup.-, CF3(CF2)7SO3.sup.-,
CF3CO2.sup.-, CH3CO2.sup.-, SCN.sup.-, and
(CF3CF2SO2)2N.sup.- may be used as an anion of the
lithium salt.

[0035] In the electrolyte solution used in the present invention, an
organic solvent included in the electrolyte solution may be used without
limitation so long as it is typically used, and typically, one or more
selected from the group consisting of propylene carbonate, ethylene
carbonate, diethyl carbonate, dimethyl carbonate, ethylmethyl carbonate,
methylpropyl carbonate, dipropyl carbonate, dimethyl sulfoxide,
acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate,
sulfolane, γ-butyrolactone, propylene sulfite, and tetrahydrofuran
may be used.

[0036] In particular, ethylene carbonate and propylene carbonate,
ring-type carbonates among the carbonate-based organic solvents, well
dissociate the lithium salt in the electrolyte solution due to high
dielectric constants as high-viscosity organic solvents, and thus, the
ring-type carbonate may be used. Since an electrolyte solution having
high electrical conductivity may be prepared when the ring-type carbonate
is mixed with low-viscosity, low-dielectric constant linear carbonate,
such as dimethyl carbonate and diethyl carbonate, in an appropriate
ratio, and thus, the ring-type carbonate, for example, may be used.

[0037] Selectively, the electrolyte solution stored according to the
present invention may further include an additive, such as an overcharge
inhibitor, included in a typical electrolyte solution.

[0038] A separator is disposed between the cathode and the anode to form a
battery structure, the battery structure is wound or folded to put in a
cylindrical battery case or prismatic battery case, and then a secondary
battery is completed when the electrolyte is injected thereinto. Also,
the battery structure is stacked in a bi-cell structure and then
impregnated with the electrolyte solution, and a secondary battery is
completed when the product thus obtained is put in a pouch and sealed.

[0039] The present invention also provides a method for preparing a porous
silicon-based electrode active material, the method comprising: mixing a
fluorine-based solution and a metal precursor solution and bringing the
mixture into contact with SiOx (0<x<2)-containing particles,
thus electrodepositing metal particles on the surface of the
SiOx-containing particles; bringing the metal
particle-electrodeposited, SiOx-containing particles into contact
with an etching solution, thus etching the SiOx-containing
particles; bringing the etched SiOx-containing particles into
contact with a metal removing solution, thus removing the metal
particles; mixing the SiOx-containing particles, from which the
metal particles have been removed, with a solution of an alkaline
hydroxide in a polar solvent; and evaporating the polar solvent from the
SiOx-containing particles, and then heat-treating the
SiOx-containing particles.

[0040] The method for preparing the electrode active material according to
one embodiment of the present invention comprises the step of mixing a
fluorine-based solution and a metal precursor solution and bringing the
mixture into contact with SiOx (0<x<2)-containing particles,
thus electrodepositing metal particles of the metal precursor solution on
the surface of the SiOx-containing particles. Herein, the
fluorine-based solution causes to the SiOx-containing particles to
donate electrons, and the metal ions in the solution are reduced by
accepting the donated electrons, and thus are electrodeposited on the
surface of SiOx-containing particles. Once the metal particles are
electrodeposited on the surface of the SiOx-containing particles,
the metal particles themselves act as catalytic sites, and thus are
continuously electrodeposited on the surface. The SiOx-containing
particles may be SiOx (0<x<2).

[0041] The fluorine-based solution that is used in the present invention
may be one or more selected from the group consisting of hydrogen
fluoride (HF), fluorosilicate (H2SiF6) and ammonium fluoride
(NH4F), and the metal precursor solution may comprise one or more
selected from the group consisting of silver (Ag), gold (Au), platinum
(Pt) and copper (Cu). The fluorine-based solution and the metal precursor
solution can be mixed with each other at a volume ratio of 10-90:90-10.
If the fluorine-based solution is used at a volume ratio of less than
10:90 in the mixing process, the amount of metal particles
electrodeposited on the surface of the SiOx-containing particles
will be small and the reaction rate will be slow, and thus the time taken
to prepare the electrode active material will be increased, and if it is
used at a volume ratio of more than 90:10 in the mixing process, the rate
at which the metal particles are electrodeposited on the surface of the
SiOx-containing particles will be very high, making it impossible to
electrodeposit metal particles having a uniform and small size on the
surface of the SiOx-containing particles.

[0042] Moreover, the amount of metal particles electrodeposited on the
surface of the SiOx-containing particles can be controlled according
to the concentration of the fluorine-based solution and the contact time
between the SiOx-containing particles and the metal precursor
solution, and the SiOx-containing particles may be used in an amount
of 0.001-50 parts by weight based on 100 parts by weight of the mixed
solution of the fluorine-based solution and the metal precursor solution.

[0043] The method for preparing the electrode active material according to
one embodiment of the present invention comprises the steps of bringing
the metal particle-electrodeposited, SiOx-containing particles into
contact with the etching solution, thus etching the SiOx-containing
particles. In this etching process, nanopores, mesopores and macropores
are formed in the SiOx-containing particles.

[0044] Etching of the SiOx-containing particles is carried out in the
following manner. The metal particles are oxidized to metal ions by
H2O2, and the SiOx-containing particles continuously enter
the solution at the interface between the SiOx-containing particles
and the metal particles while transferring electrons to the metal
particles. Also, the oxidized metal ions are reduced on the metal
particles electrodeposited on the surface of the SiOx-containing
particles. In this way, the SiOx-containing particles that have been
brought into contact with the metal particles can be continuously etched
in which occurs under the metal particles of the metal
particle-electrodeposited, SiOx-containing particles, and thus a
honeycomb-shaped porous structure can be formed on at least the surface
of the SiOx-containing particles. During the etching process, the
size of the metal particles increases, because the metal particles have a
strong tendency to agglomerate with the adjacent metal particles in the
etching solution.

[0045] The etching solution that is used in the present invention may be a
mixed solution of a hydrogen fluoride (HF) solution and a hydrogen
peroxide (H2O2) solution. The content of the hydrogen fluoride
solution in the mixed solution can vary depending on the degree of
etching, but the hydrogen fluoride (HF) solution and the hydrogen
peroxide (H2O2) solution are preferably mixed with each other
at a volume ratio of 10-90:90-10. Herein, the content of H2O2
plays an important role in the formation of mesopores in the
SiOx-containing particles, and the amount of oxidation of the metal
particles can be determined by the concentration of H2O2, so
that the concentration of metal ions can be determined. The metal
particles are oxidized to metal ions by H2O2, and the metal
ions start to adhere to specific defective sites (e.g., etched SiOx
portions), and the bottom of the SiOx-containing particles having
the metal particles attached thereto is etched, thus forming mesopores.

[0046] In addition, the etching process may be carried out for 30 minutes
to 5 hours. If the etching process is carried out for less than 30
minutes, the formation of pores in the SiOx-containing particles
will be insignificant, and if it is carried out for more than 5 hours,
the SiOx-containing particles will be excessively etched, so that
the mechanical properties of the SiOx-containing particles will be
deteriorated.

[0047] The method for preparing the electrode active material according to
one embodiment of the present invention comprises the steps of bringing
the etched SiOx-containing particles into contact with a metal
removing solution, thus removing the metal particles from the
SiOx-containing particles.

[0048] The metal removing solution that is used in the present invention
may be one or more selected from the group consisting of nitric acid
(HNO3), sulfuric acid (H2SO4) and hydrochloric acid (HCl).

[0049] In addition, the method for preparing the electrode active material
according to one embodiment of the present invention comprises the step
of mixing the SiOx-containing particles, from which metal particles
have been removed, with a solution of an alkaline hydroxide in a polar
solvent.

[0050] The alkaline hydroxide that is used in the present invention may be
one or more selected from the group consisting of LiOH, NaOH, KOH,
Be(OH)2, Mg(OH), Ca(OH)2, and hydrates thereof.

[0051] As the solvent in which the alkaline hydroxide is dissolved, any
solvent may be used, as long as it can dissolve the alkaline hydroxide
and can easily be removed. Examples of the solvent include, but are
limited to, water and an alcohol solvent. The alcohol solvent may be
ethanol or methanol.

[0052] In the step of mixing the alkaline hydroxide with SiOx,
SiOx may be used in an amount of 0.01-30 wt % based on the total
weight of the mixture. If the amount of SiOx is less than 0.01 wt %,
the initial coulombic efficiency of the anode active material will be low
because the amount of silicon and silicon dioxide formed after heat
treatment is small (SiOx is partially converted to Si--SiO2 by
heat treatment, and the content of Si--SiO2 in
Si--SiOx--SiO2 is reduced due to a low content of SiOx),
and if it is more than 30 wt o, the capacity of the anode active material
will be greatly reduced because the amount of Si--SiO2 formed after
heat treatment is large.

[0053] Furthermore, the method for preparing the electrode active material
according to one embodiment of the present invention comprises the step
of evaporating the polar solvent from the SiOx-containing particles
mixed with the polar solvent, followed by heat treatment.

[0054] Evaporating the polar solvent can be carried out at 80-120°
C. and can be carried out in an alumina boat preheated to 80-120°
C. However, evaporation of the polar solvent may also be carried out at
any temperature at which the polar solvent can be evaporated. Meanwhile,
despite evaporation of the polar solvent, the alkaline hydroxide remains
on the surface of SiOx particles.

[0055] The mixture remaining after evaporation of the polar solvent can be
heat-treated at a temperature of 750-1000° C. for 5-120 minutes.
If the heat-treatment temperature is lower than 750° C.,
crystalline silicon dioxide will not form, and if it is higher than
1000° C., a large amount of crystalline silicon will be produced
to reduce the lifetime characteristics of the secondary battery and cause
an excessive amount of energy to be consumed. In addition, the
heat-treatment time is shorter than 5 minutes, crystalline silicon
dioxide will not form easily, and if it is longer than 120 minutes, it
will not be preferable in terms of energy efficiency, because it is
significantly longer than the time required to form crystalline silicon
dioxide.

[0056] When heat treatment is carried out, the SiOx-containing
particles are disproportionated into silicon and amorphous silica
(SiO2). Specifically, oxygen in SiOx moves to the outside
(surface) to form amorphous SiO2, and silicon separated from oxygen
binds to another silicon separated from oxygen to form a silicon crystal
which is present in SiOx phase, and amorphous SiO2 is formed
mainly on the outside (surface) rather than inside the SiOx
particles. As the heat-treatment temperature or time increases, amorphous
SiOx gradually decreases and crystalline Si and crystalline
SiO2 increase.

[0057] In the present invention, the heat treatment is carried out in a
state in which the alkaline hydroxide is present on the surface of SiO
particles, thereby promoting the formation of crystalline SiO2. When
the alkaline hydroxide is not used, the crystalline peak of SiO2 is
not formed, even when the heat-treatment is carried out at the same
temperature. However, when the alkaline hydroxide is used, the intensity
of the crystalline peak of SiO2 significantly increases (around 2)
Theta=21° while a Si crystal grows. Specifically, in the prior art
in which heat treatment is carried out in order to form a composite of
SiO and carbon or in order to coat SiO with a carbon precursor or carbon,
only Si crystals grow (around 2 Theta=28.5° in the XRD), whereas
in the present invention in which heat treatment is carried out in a
state in which the alkaline hydroxide is present on the surface of
SiOx, crystalline SiO2 grows, and initial coulombic efficiency
(discharge capacity/charge capacity×100; the ratio of lithium first
charged into the silicon-based compound to the amount of lithium first
discharged from the silicon-based compound) increases. Grown crystalline
SiO2 is electrochemically inactive (non-reactive with lithium), and
SiO is divided into an electrochemically active portion (reactive with
lithium) and an electrochemically inactive portion. It is believed that,
because the molar concentration of oxygen relative to Si in the
electrochemically active portion of SiO is lower than that in SiO, the
initial coulombic efficiency increases.

[0058] In addition, the method for preparing the electrode active material
according to one embodiment of the present invention may additionally
comprise the step of filtering the above-prepared mixture.

[0059] The filtration step is carried out to remove the alkaline hydroxide
from the surface of the heat-treated SiOx-containing particles. This
step can be performed by allowing the resulting mixture to stand in
distilled water so that the alkaline hydroxide adhering to the surface of
the porous silicon-based electrode active material is removed.

[0060] Furthermore, the method for preparing the electrode active material
according to one embodiment of the present invention may additionally
comprise the step of coating the surface of the electrode active material
with conductive carbon, wherein the amount of conductive carbon coating
the surface of the electrode active material may be 1-30 wt % based on
the total weight of the silicon-based electrode material. If the amount
of conductive carbon used in the coating is less than 1 wt %, a uniform
coating layer will not form, so that the electrical conductivity of the
electrode active material will be reduced, and if it is more than 30 wt
%, an additional irreversible reaction will occur due to the conductive
coating layer, thus significantly reducing the discharge capacity of the
battery.

[0061] Hereinafter, the present invention will be described in further
detail with reference to the preferred examples. It is to be understood,
however, that these examples are for illustrative purposes only and are
not intended to limit the scope of the present invention.

Example 1

Preparation of Porous Silicon-Based Electrode Active Material 1

[0062] Electrodeposition of Ag on Surface of Silicon Monoxide

[0063] 300 ml of a solution of 10 mole % hydrogen fluoride (HF) was mixed
with 300 ml of a solution of 10 mM silver nitrate (AgNO3) for 10
minutes. 2 g of silicon monoxide (SiO) was added to and mixed with the
mixed solution of fluoride hydrogen and silver nitrate for 5 minutes,
after which the mixture was filtered, washed and dried, thereby preparing
Ag particle-electrodeposited silicon monoxide.

[0064] Chemical Etching

[0065] 200 ml of a solution of 5 mole % hydrogen fluoride and 100 ml of a
solution containing 1.5 wt % of hydrogen peroxide (H2O2) were
mixed with each other for 10 minutes. The Ag particle-electrodeposited
silicon monoxide was added to and mixed with the etching solution
consisting of the hydrogen fluoride/hydrogen peroxide mixture for 30
minutes, after which the resulting mixture was filtered, washed and
dried, thereby preparing porous silicon monoxide.

[0066] Removal of Ag

[0067] 100 ml of 60 mole % nitric acid (HNO3) was heated to
50° C., and then the above-prepared porous silicon monoxide was
added thereto and the mixture was mixed for 2 hours. Then, the mixture
was filtered, washed and dried, thereby preparing porous silicon monoxide
from which Ag was removed.

[0068] Mixing of Alkaline Hydroxide and Silicon-Based Material

[0069] 1 g of the above-prepared porous silicon monoxide was added to a
solution of 50 mg of sodium hydroxide in ethanol and stirred for 10
minutes or more.

[0070] Solvent Evaporation and Heat Treatment

[0071] The above-prepared solution containing porous silicon monoxide and
sodium hydroxide was placed in an alumina boat that has been heated to
80-120° C., and ethanol was evaporated from the solution in the
alumina boat. After the solvent had completely evaporated, the alumina
boat containing the porous silicon monoxide/sodium hydroxide mixture was
placed in a quartz tube furnace in which the mixture was then
heat-treated at 800° C. for 5 minutes in an argon atmosphere.
Then, the quartz tube furnace was cooled to room temperature, thereby
preparing a porous silicon-based electrode active material.

[0072] Immersion in Solvent, Followed by Filtration

[0073] The porous silicon-based electrode active material was recovered
from the alumina boat and immersed in distilled water, after which the
solution was filtered, thereby removing sodium hydroxide from the porous
silicon-based electrode active material.

Example 2

Preparation of Porous Silicon-Based Electrode Active Material 2

[0074] An electrode active material was prepared in the same manner as
Example 1, except that the Ag particle-electrodeposited silicon monoxide
was added to and mixed with the etching solution consisting of the
hydrogen fluoride/hydrogen peroxide mixture for 5 hours.

Example 3

Preparation 3 of Porous Silicon-Based Electrode Active Material

[0075] An electrode active material was prepared in the same manner as
Example 1, except that the heat treatment was carried out for 120
minutes.

Example 4

Preparation 4 of Porous Silicon-Based Electrode Active Material

[0076] An electrode active material was prepared in the same manner as
Example 1, except that the Ag particle-electrodeposited silicon monoxide
was added to and mixed with the etching solution consisting of the
hydrogen fluoride/hydrogen peroxide mixture for 5 hours and that the heat
treatment was carried out for 120 minutes.

[0077] 20 g of the porous silicon-based electrode active material prepared
in Example 1 was introduced into a rotary tube furnace, and argon gas was
supplied into the furnace at a rate of 0.5 L/min, and then the internal
temperature of the rotary tune furnace was increased to 800° C. at
a rate of 5° C./min. While the rotary tube furnace was rotating at
a speed of 10 rpm, the electrode active material was reacted for 3 hours
while argon gas and acetylene gas were supplied to the furnace at rates
of 1.8 L/min and 0.3 L/min, respectively, thereby preparing a porous
silicon-based electrode active material having a conductive carbon
coating layer formed thereon. The carbon content of the conductive carbon
coating layer was 10 wt % based on the weight of the electrode active
material.

[0078] Porous silicon-based electrode active materials having a conductive
carbon coating layer formed thereon were prepared in the same manner as
Example 5, except that 20 g of the porous silicon-based electrode active
materials prepared in Examples 2 to 4 was introduced into the rotary tube
furnace. The carbon content of each of the conductive carbon coating
layers was 10 wt % based on the weight of each of the electrode active
materials.

Example 9

Fabrication of Secondary Battery

[0079] The electrode active material, prepared in Example 1, was used as
an anode active material, acetylene black was used as a conductive
material, and polyvinylidene fluoride (PVdF) was used as a binder, and
these were mixed with each other at a weight ratio of 88:2:10. The
mixture was dissolved in the solvent N-methyl-2-pyrrolidone, thereby
preparing slurry. The prepared slurry was applied to one surface of a
copper current collector to a thickness of 65 μm and the resulting
structure was dried, rolled, and punched to a desired size, thereby
fabricating an anode.

[0080] Meanwhile, ethylene carbonate and diethyl carbonate were mixed with
each other at a volume ratio of 30:70 to prepare a non-aqueous
electrolyte solvent, and LiPF6 was added to the non-aqueous
electrolyte solvent, thereby preparing a non-aqueous electrolyte of 1M
LiPF6.

[0081] A lithium metal foil was used as a counter electrode. A polyolefin
separator was interposed between the anode and the counter electrode, and
then the electrolyte was injected into the resulting structure, thereby
fabricating a coin-type battery.

Examples 10 to 16

[0082] Secondary batteries were fabricated in the same manner as Example
9, except that each of the electrode active materials prepared in
Examples 2 to 8 was used the anode active material.

Comparative Example 1

[0083] A secondary battery was fabricated in the same manner as Example 9,
except that non-porous silicon monoxide was used as the anode active
material.

Comparative Example 2

[0084] A secondary battery was fabricated in the same manner as Example 9,
except that a conductive carbon coating layer was formed on non-porous
silicon monoxide in the same manner as Example 5.

[0085] The shape and crystal structure of the porous silicon-based
electrode active material prepared in Example 1 of the present invention
were analyzed by scanning electron microscopy (SEM) and X-ray diffraction
(XRD), and the results of the analysis are shown in FIG. 1.

[0086] As can be seen in FIG. 1, the porous silicon-based electrode active
material prepared in Example 1 has a large number of pores (see FIG. 1a)
and includes crystalline silicon, amorphous SiOx and crystalline
SiO2 (cristoballite) (see FIG. 1b).

Test Example 2

Analysis of Lifetime and Thickness Change Rate

[0087] In order to examine the lifetime characteristics and thickness
change rates of the secondary batteries fabricated in Examples 9 to 16
and Comparative Examples 1 and 2, the following tests were carried out.

[0088] The lifetime of each of the batteries was measured by repeating the
charge/discharge cycles at 0.5 C after the second cycle and was expressed
as the ratio of the 49th cycle discharge capacity to the first-cycle
discharge capacity.

[0089] Each of the secondary batteries was disassembled after the 50th
cycle charge, and the thickness of the electrode in each battery was
measured. Then, the difference in thickness of the measured electrode
from that of the electrode measured before charging was determined and
expressed as the thickness change rate of each battery.

[0090] Table 1 below shows the porosity, lifetime and thickness change
rate of each of the secondary batteries fabricated in Examples 9 to 16
and Comparative Examples 1 and 2.

[0093] As can be seen in Table 1 above, the lifetime of the secondary
batteries fabricated in Examples 9 to 16 was increased by 2-14% compared
to that of the secondary batteries fabricated in Comparative Examples 1
and 2, and the thickness change rate thereof was different by 14-153%
from that of the secondary batteries fabricated in Comparative Examples 1
and 2. Thus, it can be seen that, because the electrode active materials
prepared in the Examples of the present invention include not only
oxygen, unlike Si electrode active materials, but also a large number of
pores, the lifetime and swelling characteristics of the batteries were
greatly improved.

[0094] In order to examine the charge/discharge characteristics and
initial coulombic efficiencies of the secondary batteries fabricated in
Examples 9 to 16 and Comparative Examples 1 and 2, the following tests
were carried out.

[0095] To examine the charge/discharge characteristics of the secondary
batteries fabricated in Examples 9 to 16 and Comparative Examples 1 and
2, each of the batteries was charged with a constant current to 5 mV, and
then charged until the current reached 0.005 C at 5 mV.

[0096] Table 2 below shows the discharge capacity, charge capacity and
initial coulombic efficiency of each of the secondary batteries
fabricated in Examples 9 to 16 and Comparative Examples 1 and 2.

[0097] As can be seen in Table 2 above, the discharge capacities of the
secondary batteries fabricated in Examples 9 to 16 were somewhat lower
than that of the secondary battery fabricated in Comparative Example 1,
but the initial coulombic efficiencies thereof were at least about 8%
higher than that of the secondary battery fabricated in Comparative
Example 1. In addition, the initial coulombic efficiencies of the
secondary batteries fabricated in Examples 13 to 16 were at least about
5% higher than that of the secondary battery fabricated in Comparative
Example 2. Meanwhile, the initial coulombic efficiencies of the secondary
batteries of Examples 13 to 16 and Comparative Example 2 were increased
by about 4%, because of formation of the carbon coating layer.

[0098] As described above, the electrode active material according to the
embodiment of the present invention includes pores formed on the surface
and inside of a silicon oxide, and thus a secondary battery comprising
the same has a high capacity. In addition, the change in the volume of
the electrode active material of the present invention during
charge/discharge cycles can be efficiently controlled, and thus the
lifetime characteristics of the battery can be improved. Furthermore, the
initial coulombic efficiency of the electrode active material of the
present invention is increased over that of prior silicon-based electrode
active materials, and the electrode active material can be prepared in
large amounts by a simple process. Accordingly, the electrode active
material of the present invention is useful as an electrode active
material for a secondary battery.